A medical diagnostic imaging apparatus is an apparatus for imaging (fluoroscopic images, tomographic images, blood flow images, and the like) information in the body of a subject. Examples of the medical diagnostic imaging apparatus include an X-ray diagnosis apparatus, an X-ray CT apparatus, an MRI apparatus, and an ultrasonic diagnosis apparatus.
For example, an X-ray diagnosis apparatus delivers X-rays to a subject arranged between an X-ray source and an X-ray detecting unit that are oppositely arranged. The X-ray diagnosis apparatus detects X-rays passing through the subject and generates a medical image based on the detection result.
An angiosystem is known as an example of the X-ray diagnosis apparatus. For example, an angiosystem is used for seeing through the vascular morphology and the state of blood flow of a subject into which a contrast material is injected. An operator inserts a catheter and the like into a blood vessel of the subject while observing a fluoroscopic image thereof. A radiodiagnosis technique such as an X-ray angiosystem or a CT angiosystem (CT angiography, CTA) can be applied to a diagnosis, a treatment action, drainage, supply, and the like as interventional radiology (IVR).
Some X-ray diagnosis apparatuses such as an angiosystem can implement automatic brightness control. The automatic brightness control is processing for optimizing the brightness of a fluoroscopic image. Hereinafter, the automatic brightness control may be referred to as the automatic brightness control (ABC). An X-ray diagnosis apparatus stores therein a set value of a brightness level used for the ABC in advance. The brightness level of a fluoroscopic image is adjusted based on the set value and is reflected in the clarity or sharpness of the image.
More specifically, in the ABC, the X-ray diagnosis apparatus acquires a brightness level value (for example, the average value of the brightness level) of the fluoroscopic image based on transmitted X-rays obtained through X-rays irradiation. The X-ray diagnosis apparatus reads out the set value stored in advance, and compares it to the brightness level value of the fluoroscopic image. In a case where the brightness level value is different from the set value, the X-ray diagnosis apparatus changes fluoroscopy radiation conditions (imaging condition) in fluoroscopy so that the brightness level value is equal to the set value. The fluoroscopy radiation conditions include the irradiation time of X-rays (s) and the tube current (A), or the product of the irradiation time by the tube current (mAs), and the tube voltage (kV).
An X-ray imaging technique called partial fluoroscopy is also known. In partial fluoroscopy, fluoroscopy is performed in a relatively large area, and thereafter, a smaller region of interest (ROI) is set. Then a fluoroscopic image (sometimes referred to as a “partial fluoroscopic image”) in the region of interest is synthesized with a last image hold (LIH) image and the like, and displayed.
An LIH image is a static image representing the state of the periphery (background) of the region of interest for partial fluoroscopy, and is the last image (frame) in the fluoroscopy before the partial fluoroscopy. The partial fluoroscopy employs such an LIH image and the like as the background and continues performing fluoroscopy only in an area particularly focused, so that X-rays are not delivered to a portion that does not need to be irradiated, thereby reducing the dose of radiation exposure of the subject.